Semiconductor integrated circuit manufacturing technologies utilize many single-wafer thermal, deposition, and plasma etch clean process steps through the chip fabrication process flows. Semiconductor wafer temperature and overall process uniformity are important requirements in semiconductor manufacturing equipment in order to maximize device manufacturing yield. In particular, precision wafer temperature uniformity and repeatability are essential requirements in thermal processing equipment including rapid thermal processing (RTP) systems. For instance, rapid thermal anneal (RTA), rapid thermal oxidation (RTO), and rapid thermal chemical-vapor deposition (RTCVD) processes must provide dynamically uniform and repeatable wafer temperatures with within-wafer temperature non-uniformities and wafer-to-wafer temperature variations of less than .+-.5.degree. C. in most thermal processes for sub-half-micron semiconductor IC production. Moreover, chemical-vapor deposition (CVD) processes must provide high-quality material layers (such as polysilicon, amorphous silicon, epitaxial silicon, silicon nitride, silicone dioxide, etc.) with uniform and repeatable material layer thicknesses. This places a demand for not only wafer temperature uniformity and repeatability control, but also reactant mass transport uniformity across the semiconductor wafer or any other substrate (e.g., then-film head, flat-panel display, etc.).
One effective method to establish improved wafer temperature uniformity and also improved reactant mass transport uniformity in single-wafer fabrication equipment is wafer rotation during wafer processing. For instance, wafer rotation in an RTP equipment can result in axisymmetrical wafer temperature distribution with improved temperature uniformity and enhanced repeatability. Moreover, wafer rotation in RTCVD and single-wafer plasma-enhanced CVD (PECVD) systems is effective in establishing an axisymmetrical reactant mass transport distribution profile over the wafer with improved deposition process uniformity, including uniform material layer thickness distribution.
Another application of wafer rotation in semiconductor manufacturing equipment is deposition rate enhancement in CVD Rotating Disk Reactor (RDR) systems. The typical wafer rotation speeds for temperature and process uniformity improvements are in the range of 20 RPM to 200 RPM (RPM=Revolutions per Minute). The RDR CVD systems, however, employ much higher wafer rotation speeds, typically in the range of 500 to over 1500 RPM. These relatively high rotation speeds are required in order to increase the material deposition rate in certain CVD process applications by reducing the boundary layer (stagnant layer) thickness, particularly for mass-transport-limited deposition processes.
The wafer rotation methods in single-wafer manufacturing equipment typically employ external rotation motors in conjunction with ferrofluidic feedthroughs. A ferrofluidic feedthrough provides a rotatable shaft wherein one end of the shaft can be placed within the process chamber (e.g., vacuum process chamber) while the other end of the shaft can be placed on the atmospheric side outside the process chamber. An electrical motor applies rotation to the ferrofluidic feedthrough shaft outside the fabrication equipment process chamber. This rotation is performed through direct mechanical transfer through to the ferrofluidic feedthrough shaft terminal located inside the fabrication equipment process chamber. The ferrofluidic shaft is connected to the wafer holder, or wafer susceptor or any wafer support structure, resulting in wafer rotation. This wafer rotation configuration has several disadvantages in terms of equipment reliability and performance.
Generally, ferrofluidic feedthroughs present a problem due to their limited life in operation. Wafer rotation mechanisms with ferrofluidic feedthroughs can also generate particulates in the process chamber, resulting in a reduction of the manufacturing yield due to an increase in chip defect density.
In vacuum or low-pressure CVD applications, the ferrofluidic feedthrough can also cause vacuum integrity problems and possible vacuum base pressure degradation over extended operation.
In RTP systems, the use of ferrofluidic feedthroughs for wafer rotation also has some obvious disadvantages and constraints. For instance, a typical ferrofluidic rotation assembly will consume the much needed space below the wafer backside. As a result, this type of wafer rotation eliminates availability of an access port facing the wafer backside, placing a serious constraint on equipment design.
An alternative method for wafer rotation was developed in the Microelectronics Manufacturing Science and Technology (MMST) program at Texas Instruments Incorporated for use in RTP systems. Both inventors of the present invention, Mehrdad M. Moslehi and Yong Jin Lee, were members of the MMST team who developed the Advanced-Vacuum Processor RTP or AVP RTP system. The MMST RTP rotation assembly was designed based on the use of two multipolar magnetic wheels, as schematically shown in FIG. 1. The internal multipolar magnetic field contains an even number of preferably radially magnetized magnets with alternating poles. These magnets are mounted on a soft magnetic material ring composed of a permeable magnetic material such as nickel-plated iron, 400 series stainless steel, or 1018 steel. The internal multipolar magnet wheel is attached to the wafer holder and a quartz or metallic liner (or wafer support structure) supported on a circular or ring-shaped mechanical bearing assembly. The rotation motor rotates the external magnet wheel through a gear coupling mechanism. Rotation of the external magnet wheel results in rotation of the internal magnet wheel due to the strong magnetic field coupling between the two sets of multipolar permanent magnets through the process chamber well. Rotation of the internal multipolar magnet wheel results in wafer rotation through rotation of the wafer support assembly.
The multipolar magnet rotation has an important advantage over the ferrofluidic feedthrough rotation since it does not employ a ferrofluidic feedthrough. As a result, the multipolar rotation assembly is a more reliable method of wafer rotation in semiconductor manufacturing equipment, including single-wafer vacuum and atmospheric fabrication equipment. Another advantage of the multipolar rotation design is the fact that it does not need to occupy the spaces directly above or below the semiconductor wafer. Thus, the regions directly above and below the wafer can be used for implementation of process energy sources as well as other fabrication equipment components and subassemblies.
However, this multipolar rotation design, like the other prior wafer rotation methods and devices, still presents the disadvantage of having mechanically moving parts outside the fabrication process chamber in the manufacturing clean room. These mechanical parts, such as the motor, motor shaft, and rotating gears, can be sources of equipment reliability and downtime problems as well as generators of particulates. Generation of particulates due to external mechanical motion and rotational friction can degrade the clean room cleanliness and class around the fabrication equipment.